Composite gas separation modules are commonly used to selectively separate a particular gas from a gas mixture. These composite gas separation modules may be made of a variety of materials, including, for example, polymers and metallic composites. While these composite gas separation modules can provide effective and cost efficient alternatives for the separation of gases at low temperature process conditions, they often are unsuitable for use in high temperature and pressure gas separation processing.
Certain types of gas separation modules are disclosed in the prior art that are intended for use in high temperature gas separation applications and that have structures consisting of a selective gas permeable metallic membrane mounted on the surface of a porous substrate. For instance, US Patent Publications 2004/0237780 and 2009/0120287 disclose gas separation systems for the selective separation gases. Both teach that the gas separation system is made by first depositing a gas-selective metal by electroless plating which is generally palladium onto a porous substrate followed by abrading the resultant coated substrate and, thereafter, depositing a second layer of a gas-selective metal which is also generally palladium upon the coated polished porous substrate. In US 2004/0237780, the intermediate step of abrading or polishing of the coated substrate is used to remove unfavorable morphologies from the surface of the coated substrate. In US 2009/0120287, the intermediate abrading step is used for the purpose of removing a substantial portion of the first deposited material to provide a thinner dense gas selective membrane. These publications do not address the problems associated with attempting to deposit a layer of silver onto a layer of palladium.
The problem of silver agglomeration is well-known. “The Inhibition of Silver Agglomeration by Gold Activation in Silver Electroless Plating,” Cha et al., Journal of the Electrochemical Society (2005), C388-C391, describes silver agglomeration as an obstacle to obtaining thin silver films. A layer of gold was used as an activation material for the substrate and thereafter, silver film was electrolessly deposited on to the gold-activated substrate. Silver has a different crystal structure than palladium and if one tries to plate silver on palladium, the silver will plate on itself and form islands on the surface of the palladium.
Another approach to overcoming the problem of silver agglomeration is to chemically activate the surface to be plated prior to deposition of the silver. One such method of chemical activation is disclosed in U.S. Pat. No. 7,175,694, wherein an oxidized stainless steel tube was surface activated by immersing the tube in aqueous baths of SnCl2 and PdCl2 prior to sequential application of layers of palladium and silver. This method of activation consumes large amounts of water and generates significant volumes of aqueous waste that requires treatment before discharge and also leaves residues of tin and chloride ions, which need to be removed.
Another method of activation of a palladium surface utilizes palladium acetate in chloroform solution and involves evaporation, drying and decomposition of the acetate followed by reduction to palladium metal seeds.
Since chemical activation methods, such as those described above, involve multiple steps, they tend to be expensive and time consuming, in addition to generating waste products which need to be treated.
A non-chemical method for activating the surface of metals is disclosed in U.S. 2011/0232821. However, the disclosed method employs a different surface roughness and morphology than employed in the present inventive method.
Therefore, there is a need in the art for an efficient and cost effective method to prepare a palladium-silver alloy gas separation membrane system in which the silver can be uniformly deposited on a layer of palladium without the need for chemical activation or the need to include a layer of gold as an activating material.